1. Field of the Invention
This invention relates to an apparatus for reading and/or recording digital information on an optical disk and, more particularly, to an improvement for reducing residual offset in tracking error signals detected by a pair of light detectors situated symmetrically with respect to the center of a spiral or coaxial track on the optical disk.
2. Description of the Related Art
An optical disk on which spiral or coaxial tracks are arranged to record bits of digital information has been developed as an external memory media for digital computers. There are many different formats for the tracks, but a single track (one circulation of the disk) typically includes seventeen sectors. Typical sector formats which are now under discussion for industry standards are shown in FIGS. 1a and 1b. In the format of FIG. 1a, a groove, called a pre-groove, is provided between adjacent tracks. Data bits are recorded on the area between the pre-grooves, and recording in this format is called "on-land recording". In contrast, FIG. 1b shows a format used for "in-groove" recording. In the format of FIG. 1b, a pre-groove is provided on a portion where the bits are to be written. Recording using this format is called "in-groove recording".
In either of the above formats, each sector typically stores 1360 bytes. Each sector is broken into a data area 11, storing 1307 bytes, and an ID (identification of the sector) area 12 storing 53 bytes. The ID area 12 is made up of a preformat area 12-1, and a mirror mark 12-2 which stores no data and has no groove, but takes up one byte space. The data bits of the pre-format area 12-1 are installed in pits formed along with the grooves on the disk when the disk is manufactured. Further explanation of the mirror-mark 12-2 follows below.
The data area 11 is free of data until the data bits are written thereon during recording. The "dashed" circles in FIGS. 1a and 1b indicate a "potential" sequence of data bits to be written on the data area 11. Vacant zones 11' and 11" are provided at either end of a sequence of data bits recorded in the data area 11. The vacant zones are always free of data and cause gaps as long as 20 microseconds (referred to hereinafter as .mu.s).
The radial pitch of the tracks of these disks is small, typically 1.6 micrometers (referred to hereinafter as .mu.m) and the rotation of the disk may be eccentric (varying as much as several hundred .mu.m). For a light spot SP1 from a laser to consistently trace the center of the track, a servo mechanism is provided. A typical configuration of a prior art optical pick up and its servo mechanism is schematically shown in FIG. 2. A light produced by a semiconductor laser 21 is focussed by a collimator lens 22 and an object lens 24 to produce a light spot or light beam SPl on the track center. Light reflected from the spot SPl is further reflected by a polarization light splitter 23 to focus a spot SP2 onto a light detector 3, which includes two symmetrical photo detector elements A and B. The light detector 3 is situated so that the center line of the track is focussed to coincide with the border of the two elements A and B. Elements A and B sense the amount of light reflected from the disk. If the light beam is focussed on the track center line, elements A and B will receive equal amounts of reflected light and output a value indicative of this. If the elements A and B sense different amounts of reflected light, this indicates that the light beam is not focussed on the center of the track. A difference between the output levels of the two elements A and B is detected by a differential amplifier 4. The tracking error output Ste of the differential amplifier 4 indicates the amount and direction of the tracking error, which is a deviation of the light spot SPl from the track center.
Characteristics of the tracking error signals are explained below with reference to FIGS. 1b, 2 and 3. A tracking error signal is fed back to a servo mechanism to adjust object lens 24 (FIG. 2) to center the light spot on the track. However, if, for example, the disk is skewed, the light spot will still be "offset" from the track center. FIG. 3 shows the value of the tracking error signal versus deviation of the light spot SPl from the center of the track, for a disk having no offset. The solid line (a) indicates the tracking error signal Ste at the pre-groove area 11 having no data-bits thereon. The dashed line (b) indicates the tracking error signal at the pre-format area 12-1 having pre-format bits thereon. Because there is no offset, the zero points of the two tracking error signals coincide with each other and with the center of the track. The tracking error signal Ste is negatively fed back through a servo amplifier 5 (FIG. 2) to a tracking drive 25 of the tracking servo mechanism to adjust location of the object lens 24 or the entire optical pick-up 2, so that the tracking error signal approaches zero. This is the widely used push-pull method. However, even when the spot traces the track center, if the disk is skewed, or the axis of the lens is not aligned on the center border line of the light detector 3, or other adjustments are poor, a tracking error signal may still occur. This tracking error signal is like a bias component in the signal and is called the offset. The offset is further explained with reference to FIG. 4, in which "D" indicates the offset in radial distance on the track, and Sos indicates the offset in the tracking error signal Ste. The offset is represented by the amount Sos of the tracking error signal Ste when the spot is correctly located at the center of the track. In other words the deviation "D" of the light spot SP1 from the center of the track, when the spot is servo controlled to keep the tracking error signal essentially zero, is the offset. The zero point of the tracking error signal including the offset component does not coincide with the center of the track. Therefore, a tracking servo system controlled by only the above-mentioned tracking error signal still deviates from the center of the track.
Prior art techniques to compensate for this offset and its problems are hereinafter described. One such prior art technique is disclosed by T. Kaku et al. in the U.S. Pat. No. 4,663,751 and unexamined Japanese Patent No. Sho 61-280036. In the Kaku et al. publications, the reflected light from a mirror mark produces a tracking error signal which indicates the degree of the offset, because the light spot at a pregroove area is servo controlled so that the tracking error signal approaches zero. Thus, the tracking error signal at the mirror mark is utilized for compensating the offset. However, the circuits referred to in Kaku et al. must sample a mirror mark as short as 1.5 .mu.s and handle very high frequencies of more than 5 MHz produced by the pre-format bits. Accordingly, circuits to allow this sampling, such as differential amplifiers, adder circuits and sample-hold circuits, cause considerable increases in production costs and occupy substantial space. Further, this method cannot be applied to a disk format that does not have a mirror mark.
Another prior art technique is disclosed by K. Tatsumi et al. in unexamined Japanese Patent Nos. Sho 61-8745 and Sho No. 61-13447. In these two publications, the reflected light from a flat portion between each pre-format bit is sampled and utilized as a signal which indicates a degree of offset. Circuits for sampling the portions between 5 MHz bits require higher frequency characteristics than those of the abovementioned Kaku's et al. circuits. Thus, the circuits must be very sophisticated, expensive and bulky.
Another prior art technique is disclosed by Y. Tsunoda et al. in "On-land Composite Pre-groove Method for High Track Density Recordings" presented during the Proceedings of SPIE-The International Society for Optical Engineering held on Aug. 18-22, 1986 in San Diego, Calif., at a session entitled "Optical Mass Data Storage II". In this report, Tsunoda et al. proposed utilization of wobbled pit marks in the place of the above-mentioned mirror mark. Circuits used in Tsunoda's proposal also require high frequency characteristics of several MHz. Again, the circuits must be very sophisticated, expensive and bulky.